I may not be the right person to ask because I went from 456--to 373 & now wished I had gone to 355-or 342 so I say 342
Where are you driving the most ??


. Later Larry Mc

 

4.56 with a 4 speed? That's insane man.

I do most of my driving in town and on back roads. I do occasionally take it out on the highway though. I've put my info into an rpm calculator and this is what i got @ 65 mph:

3.42 - 2837 rpm

3.73 - 3133 rpm

A difference of about 300 rpm.

I feel like going with the 3.42 is what i "should" do... and it's what all older guys (my dad and his friends) tell me to do, but I'm 24 and I want the extra performance off the line :devil: hmm...

 

Time to get out the math book

Here's a calculator to help you understand the effect of gear ratio to rpm & speed. Obviously lower gears mean faster acceleration and higher RPM at say 60mph, along with lower top end and poorer fuel economy.

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

You really have to decide that happy medium of performance and drivability. I have 3.73 behind my TH350 and 28" tall tires. At about 60 mph I'm turning around 2700 PM and don't have the beginnings of knowing my gas mileage because I haven't got set the speedo to read correctly yet. I only have about 175 miles on the car since getting it on the road. But, it gets better fuel economy than the boat (2 mpg), so I don't care.

Keep us in the loop with what you decide. -p-

 

Here's a calculator to help you understand the effect of gear ratio to rpm & speed. Obviously lower gears mean faster acceleration and higher RPM at say 60mph, along with lower top end and poorer fuel economy.

http://www.ringpinion.com/Calculators/Calc_RPM.aspx

You really have to decide that happy medium of performance and drivability. I have 3.73 behind my TH350 and 28" tall tires. At about 60 mph I'm turning around 2700 PM and don't have the beginnings of knowing my gas mileage because I haven't got set the speedo to read correctly yet. I only have about 175 miles on the car since getting it on the road. But, it gets better fuel economy than the boat (2 mpg), so I don't care.

Keep us in the loop with what you decide. -p-

Those 28" tires are helping keep down your rpms. With my 26" tires i'd be pushing 2900 vs. your 2700 @60mph.

I understand the effect of rear gear ratio and I know how they look on paper.
I just want to know how much of a performance gain i'm looking at with the 3.42 versus the 3.73.

Why don't they offer a 3.55 for the 8.5? That's what my dad has in his 8.2. I guess 3.42 isn't much of a difference... most likely wouldn't be able to feel the difference.. or would you?

 

I would have to say the 342

 

I say the 3.42 ratio but think you would like 3.55 if you have not locked yourself to the choices.

 

I have ran both the 3.08 and 3.42 gears for street use.

If you make this change you will notice some difference in both initial take off and pulling power in 4th gear.

I have never liked the 3.73 gears unless it coupled to an overdrive transmission.

 

Here's a snapshot of an excel spreadsheet I made up to compare different trannys and gear ratios. Just enter your desired tire size and MPH, and you can see what RPM you'd be at with different gears and transmissions. PM me your email address and I can send it to you if you want.

 

If you really want to wake it up then go with the 3.73's. If you drive a lot of freeway then 3.42's. With the 3.73's I had in my last 71 I would drive the highway around here and about 25 miles over to the race track. Running 55-60 no problem.

You answered your own question _ "I don't do much highway driving". There you go 3.73's
I loved the 3.73's in my 71.

I just put 4.10's in mine on Saturday. I feel like I need to grab another gear now.

 

You can get a 3.55 gear for the 8.2, but I don't think anyone offers one for the 8.5.

 

No 3.55 for 8.5

WOOPS, !! 3:73 it is...

 

No 3.55 for 8.5

 

I think that if i go with the 3.42's i will always be wondering what it would have been like with the 3.73.

I can always swap out the gears if i don't like them.

So right now it's lookin' like i'll be going with the 3.73.

I will be ordering my parts in 2 weeks so i'll let you guys know what i decide when the time comes.

 

I think you will love them.:yes::yes:

 

I too have 3.08s with 26.50" tall tires. I've been considering the same to ratios that you have listed for my 8.5" (3.42 or 3.73), only difference for me is that I have a TH350 trans. Since neither of us have an OD, our crusing rpms will be 3000rpm+ at highway speeds with the 3.73s. I'm leaning closer to the 3.42 myself.

 

4.10 with 26.5" tires at 60mph is 3100 rpm.
3.73 is 2800 rpm.
3.42 is 2600 rpm.
3.08 is 2350 rpm.

 

If your gonna do the Power Tour and run the interstates, a 3.36 with a 26" tire is the highest I'd go. My 3.08's are good for cruizing but I'd like to wake things up a bit, I'm going to a 3.36. Any higher and your gonna be buzzin' at highway speeds. And were talking 70-80 on the turnpike.
What brand gears are you going with?

 

I believe my dad used richmond gear for his rear end so i'll probably do the same. Do you have any recommendations?

I had a muncie and 255/60/15 on the rear with 3.08 then swaped to a 3.42 man it woke it up and a lot more fun on the street but not so much on the hwy

So you noticed a pretty big difference after switching to the 3.42's then?

 

I'd go 3.42 if you want to feel what it would feel like with 3.73s just put 24" tires on. :thumbsup:

But in all reality I have 3.73s in my car will be going to 4.11s or even 4.30s in a moser 9" for my big block.

 

With only around 200 rpm difference between the two I would run the 3.73's, but not my car. I currently run 4.56's but never get it on the interstate, normally cruise at the 3000 to 3500 rpm range. :2cents:

 

I had a muncie and 255/60/15 on the rear with 3.08 then swaped to a 3.42 man it woke it up and a lot more fun on the street but not so much on the hwy

 

i have 373, and wish i didnt .. i work 35 miles away and im pushing 3000plus just to keep her at 60mph. id like to drive her to work once in a while , now im looking to get an overdrive.. it just never ends.. buy ,install. re buy ...replace.. then ya have to sell all the stuff for peanuts that you bought that was wrong.. next time the car is going to be planned out on paper first..:yes:

 

I think a 3.23 would be just about the ideal compromise between highway driveability and decent performance for those of us with tire diameter in the 26-27" range, and without an overdrive. But going from a 3.08 to a 3.23 wouldn't really be worth the cost or labor, so the next step up for an 8.5" would be the 3.42.

 

I have personal seen all of these combinations and it comes down to several things.

And for reference. I did RR change on member's 69 w a 4 speed. From a 3.08 to a 8.5 // 3.42

The short tire a lot of "gurou's" don't take into consideration.

Also your 4 speed, if close ratio, first gear is a little tall and does make for sluggish takeoff.

Back to the result and change.
Note: when I say highway, I'm talking 70mph not the allusion 55mph that everyone seems to believe is what you want.
His main issue was take off and ease of drivebilty in traffic without killing the highway. The 3.42 gear was a great choice. And what I choose to be the highest gear I would choose for Highway. (without over drive)

Toss up!
If Highway is more important go with a 3.23 that will be a 10% change from your 3.08 (it's a bigger change than you realize)

If you don't mind a "little" loss of mileage and prefer the take off more, go with a 3.42 a 12% change from your 3.08

When I drove Don's car it was impressive with the 3.42, but at 70 mph you just wanted to pull another gear it did not have. I drove the car with the 3.08 and highway was awesome.

IMHO
Al

You can contact Don and ask him about his gear change, he has the 4 speed like you. http://www.stevesnovasite.com/forums...=newpm&u=18590

 

If Highway is more important go with a 3.23 that will be a 10% change from your 3.08 (it's a bigger change than you realize)

If you don't mind a "little" loss of mileage and prefer the take off more, go with a 3.42 a 12% change from your 3.08

I definitely agree that a 3.23 is a good compromise between highway cruising and street performance in a non-OD application but I am confused by your numbers, or maybe I am just misunderstanding your calculations.

10% of 3.23 is 0.323. 3.23 - 0.323 = 2.90. So if you were to take a 10% loss from a 3.23, it would be a 2.90 gear (tire diameter and trans ratios being equal)

10% of 3.08 is 0.308. 3.08 + 0.308 = 3.38. So if you were to jump 10% from 3.08, it would be a 3.38 gear (tire diameter and trans ratios being equal).

Either way, it seems to me that a 3.08 to 3.23 would be more like a 5% change, and the 3.42 gear would be more like a 10-11% change.

I just don't think it's a worth the cost to swap to 3.23 from a 3.08 unless you can do the labor yourself, or you need a new rear anyway. Too much cost for not enough gain IMO. I guess if you are determined to get every last ounce of street performance while keeping highway rpms under a strict limit, then such a small change might be worth it.

 

i say go with 3:73's but ditch the 4 speed and get an overdrive!!! :yes:

best of both worlds!!!! :yes:

 

EVENTUALLY I want to put in an LS backed by a 5 or 6 speed, but that is a lot of money that i don't have at the moment. I'm just going to have fun with what I have now and make the best of it.

 

question...

Great thread as I am going thru the same thing right now. I am sure my gear guy will know what to get but in the meantime.....

How do I know what size differential I have? This is the original 12 bolt posi unit. Currently have 4.11's and I remember way back when that the guy who did my gears needed to install some type of adapter ring or spacer of some sort in there, if that info helps. Just wondering what I got as it appears that it will affect my gear ratio options. Thanks.

 

 

3.73

I went from 3.08s to 3.73s years ago, one of the best things I did to the car, really woke it up.

If you cruise on the highway a lot, its your daily driver, or youre worried about mpgs, then go with 3.42's.

If you want better off the line performance and all around more fun without worrying about going on the highway a lot or mpgs, then get the 3.73's.

It sounds like you want performance and dont cruise much on the highway, so I'd go with the 3.73 gears.

 

I went from 3.08s to 3.73s years ago, one of the best things I did to the car, really woke it up.

If you cruise on the highway a lot, its your daily driver, or youre worried about mpgs, then go with 3.42's.

If you want better off the line performance and all around more fun without worrying about going on the highway a lot or mpgs, then get the 3.73's.

It sounds like you want performance and dont cruise much on the highway, so I'd go with the 3.73 gears.

x2 and 3.:yes::yes::yes:
I just went from 2.73 to 4.10s last Saturday.

 

Hey Now, You said the differential is a ONE legger; but no mention of putting in a Posi unit. No matter which gear ratio you choose, You need a Posi track unit. Just adding 42's or 73's will not make you faster around town or at the drag strip. I had a one legger that would SMOKE one tire for over a 1/4 mile; and that was taking off in 2nd gear... Get an Auburn Posi with 3.42's and Enjoy Best of Both worlds. Respectfully, "Bill's" II

 

That's the main reason for rebuilding it. I wanted a posi but while i have it apart i might as well put in some lower gears at the same time.

I have heard the auburn units are not the best. But i've never had one so i don't know.

I was thinking of going with either an eaton truetrac or a yukon duragrip.